1. Field of the Invention
The present invention relates to elastomeric yarn knitting where yarn packages in a creel are unwound and presented to a circular knitting machine. The present invention also relates to computer-controlled feeding systems that compensate for the variable package relaxation or unwind tension level of elastomeric bobbins.
2. Description of the Related Art
Elastomeric yarn is stored, by winding yarn onto a tube to form a cake or bobbin, sometimes referred to as a yarn “package.” Elastomeric yarn typically comprises a continuous filament that has a break elongation in excess of 100% and which, when stretched and released retracts quickly and forcibly to substantially its original length. Such fibers include, but are not limited to, rubber, polyether polyester, and any elastane such as Lycra® or Elaspan®. Elastomeric yarn may have these stretch fibers covered with other, non-elastomeric fibers, sometimes called “hard yarn” or “companion yarn,” such as cotton or nylon.
Elastomeric yarns are typically spun at high speeds and stretched during winding in order to build a stable package and to guarantee that the wound yarn will not fall off of the tube. This process can leave residual tension within the bobbin. A given length of elastomeric yarn may have a relaxed length L1 as schematically shown in FIG. 4. As the yarn is wound it elongates to length L2 (i.e. L2>L1) as schematically shown in FIG. 5. When the yarn is taken off the package, the same given amount of the wound yarn relaxes and returns to a relaxed length L1 as shown in FIG. 4.
For example, an elastomeric yarn bobbin may have an outside yarn circumference of 25 cm. Four revolutions of the stretched yarn on the outside layers of the package would have a stretched length L2 of 25×4=100 cm. When unwinding four revolutions of the package and relaxing the thread line to zero tension, the final unwound length L1 is less than 100 cm, such as, for instance, 90 cm. The package relaxation percentage can be expressed by the equation:PRL%=((LS−LR)/LR)×100%In this equation, LS is the stretched length, LR is the relaxed length and the PRL is the package relaxation (PRL), which is expressed as a percentage. The package relaxation of the yarn on the bobbin in this example is calculated as: ((100−90)/90)×100%=11.1%. The amount of relaxation that occurs with any given amount of the yarn as the yarn is wound is not necessarily constant. Thus, there is a need to accommodate the package relaxation when processing a given material.
Machines for warping or beaming of elastomeric bobbins have been adapted to compensate for the package relaxation of a given elastomeric yarn. One conventional beaming apparatus includes a creel with elastomeric yarn packages positioned on and driven by drive rolls. The creel can contain 1100-1500 bobbins, the threadlines of which are unwound simultaneously onto big metal beams. The yarn is fed as one threadsheet comprised of individual thread lines. Normally, three, six or even nine beams are created from one creeling of elastomeric bobbins. The beams are typically used in a warp knitting machine as a set. For example, a set of three beams of elastomeric material may be combined with a set of three beams of nylon hard yarn to knit an elastified fabric. The characteristics of the beams within one set should be identical. Otherwise, the weight and circumference variation among the individual beams will lead to unacceptable tension-variations within the knitted fabric. Adjusting the creel speed or the beam speed based upon an assumed or actual package relaxation profile within the bobbin reduces of prevents these variations. Therefore, compensation of package relaxation is a basic requirement in warping, and is done automatically while unwinding elastomeric bobbins.
Currently, circular knitting machines and techniques do not compensate for package relaxation of elastomeric yarn bobbins.
Circular knitting of elastified fabrics usually requires a number of bobbins of elastomeric yarn in the creel of the machine. Circular knitting machines are well known in the art and can typically handle from a few bobbins up to 144 elastomeric bobbins. The majority of known circular knitting machines that are used handle 48 to 96 bobbins.
A majority of the devices used for unwinding the individual bobbins in circular knitting machines are “Fournisseure” type elastomeric rollers, such as the MER-2 from Memminger-IRO Gmbh of Dornstetten, Germany. The MER-2 can hold and run one to four elastomeric yarn bobbins. The bobbins of elastomeric yarn are surface driven on two small metal drive rolls. The unit itself is connected to a narrow, centrally-driven belt.
One manner of adjusting the speed of the drive belt in Fournisseure-type rollers is by adjusting the diameter of a belt in an expandable pulley, also known as a quality adjustment pulley or quality wheel. Expandable pulleys are well known and typically have a top and bottom ring or disk with which may have at least one graduated inside surface or other means for moving a belt in an axial direction. Turning the disks will position them closer or further from each other, which forces the belt to move along the axis of the pulley. For example, as the disks are moved toward each other, the belt is forced outward along the radius of the disks which increases the gauge, i.e. the diameter of the belt in the expandable pulley. Normally, a single known gauge is used for a particular circular knitting machine and/or for a particular fabric style. The pulley, which is driven by a central motor of the circular knitting machine, turns a belt that, in turn, turns the unwinding device or devices.
Traditional circular knitting machines often have a single central motor that drives a dial, a fabric take-off roll, and a number of yarn feeder systems. The dial registers and presents the yarns from the bobbins to knitting operation positions. One belt and pulley system drives the yarn feeder systems that feed the elastomeric yarn off of the elastomeric roller. The expandable pulleys are adjusted to one fixed setting for a specific fabric style or quality and, therefore, no corrections are ever made for the package relaxation profile.
Several circular knitting machine manufacturers have replaced expandable pulleys with small and independent computer controlled brushless motors or step motors to control the speed of the drive belts in the creel. A large motor drives the dial with the needles and the fabric take-off roll. Small separate motors drive the belts that directly run the individual take-off devices for the elastomeric yarns and, in some cases, the companion yarns. However, the yarn feed velocity is still set at one fixed speed setting within one particular creeling of elastomeric bobbins or fabric style.
The effect of package relaxation on the final circular knit product has not been well understood within the art. Knitting tension measurements may be performed, but are not always accurate as they are typically performed at only a few positions (typically 2-4 positions). Apart from the accuracy of the applied tension device, the tension varies from position to position on the circular knit machine and may be caused by variations in the elastomeric yarn or the circular knit machine. The measured data is not reliable because often only a few measurements are performed at only a few locations.
Additionally, measurements and quality checks in circular knitting are conventionally done only when starting up a creel of full bobbins. Fabric knitters and fabric manufactures do not perform measurements during knitting or after finishing each and every fabric roll that is being made on one creeling of elastomeric yarn bobbins.
A measurement computer, such as the LMT6 manufactured by Memminger IRO GmbH of Dornstetten, Germany, can be used at the start up of the circular knitting process to measure the speed at which the yarn is fed. The LMT6 is programmed with a package relaxation level (typically based on only one data point and only from the outside layers of the elastomeric yarn bobbin). For example, an LMT6 user often applies one assumed package relaxation value of, for example, 10% for all elastomeric yarns. This assumed value fails to account for variations in the package relaxation profiles between elastomeric yarn decitexes, between different package sizes from one yarn supplier to the other, and between packages from the same supplier. Furthermore, no compensation is made for the package relaxation profile within one bobbin of elastomeric yarn as it is unwound. The LMT6 then performs predetermined calculations and prints the data. The LMT6 measurement computer, however, does not control the drive systems of the circular knitting machines.
The elastomeric yarn content data that is generated by the LMT6 device is typically based on measurements performed only at the start-up of each knitting process. The speeds of the various motors on the circular knit machine are set to a constant value. Any variation in the package relaxation during unwinding is not considered, and is therefore left uncompensated.
Failing to compensate for the relaxation profile of each bobbin of elastomeric yarn in circular knitting can lead to fabrics of varying characteristics and lower quality. Methods and equipment to determine and compensate for the package relaxation profile of an elastomeric yarn in a circular knitting machine would overcome these deficiencies.